The present invention relates to the use of metallocene catalysts in the production of polyolefins, particularly polypropylene, and the ability to vary certain properties of the polymer products by varying the structure of the catalyst. In particular, it has been discovered that changes in the structure and composition of a bridge linking two cyclopentadienyl groups in the metallocene catalyst changes the melting points and the molecular weights of the polymer products.
The use of metallocenes as catalysts for the polymerization of ethylene is known in the art. German patent application 2,608,863 discloses a catalyst system for the polymerization of ethylene consisting of bis(cyclopentadienyl)-titanium dialkyl, an aluminum trialkyl and water. German patent application 2,608,933 discloses an ethylene polymerization catalyst system consisting of zirconium metallocenes of the general formula (cyclopentadienyl).sub.n Zr Y.sub.4-n, wherein Y represents R.sub.1 CH.sub.2 AlR.sub.2, CH.sub.2 CH.sub.2 AlR.sub.2 and CH.sub.2 CH (AlR.sub.2).sub.2 wherein R stands for an alkyl or metallo alkyl, and n is used a number within the range 1-4; and the metallocene catalyst is in combination with an aluminum trialkyl cocatalyst and water.
The use of metallocenes as a catalyst in the copolymerization of ethylene and other alpha-olefins is also known in the art. U.S. Pat. No. 4,542,199 to Kaminsky, et al. discloses a process for the polymerization of olefins and particularly for the preparation of polyethylene and copolymers of polyethylene and other alpha-olefins. The disclosed catalyst system includes a catalyst of the formula (cyclopentadienyl).sub.2 MeRHal in which R is a halogen, a cyclopentadienyl or a C.sub.1 -C.sub.6 alkyl radical, Me is a transition metal, in particular zirconium, and Hal is a halogen, in particular chlorine. The catalyst system also includes an aluminoxane having the general formula Al.sub.2 OR.sub.4 (Al(R)--O).sub.n for a linear molecule and/or (Al(R)--O).sub.n+2 for a cyclic molecule in which n is a number from 4-20 and R is a methyl or ethyl radical. A similar catalyst system is disclosed in U.S. Pat. No. 4,404,344.
U.S. Pat. No. 4,530,914 discloses a catalyst system for the polymerization of ethylene to polyethylene having a broad molecular weight distribution and especially a bimodal or multimodal molecular weight distribution. The catalyst system is comprised of at least two different metallocenes and an alumoxane. The patent discloses metallocenes that may have a bridge between two cyclopentadienyl rings with the bridge serving to make the rings stereorigid. The bridge is disclosed as being a C.sub.1 -C.sub.4 alkylene radical, a dialkyl germanium or silicon, or an alkyl phosphine or amine radical.
European Patent Application 0185918 discloses a stereorigid, chiral metallocene catalyst for the polymerization of olefins. The bridge between the cyclopentadienyl groups is disclosed as being a linear hydrocarbon with 1-4 carbon atoms or a cyclical hydrocarbon with 3-6 carbon atoms. The application discloses zirconium as the transition metal used in the catalyst, and linear or cyclic alumoxane is used as a co-catalyst. It is disclosed that the system produces a polymer product with a high isotactic index.
It is known by those skilled in the art that polyolefins, and principally polypropylene, may be produced in various forms: isotactic, syndiotactic, atactic and isotactic stereoblock. Isotactic polypropylene contains principally repeating units with identical configurations and only a few erratic, brief inversions in the chain. Isotactic polypropylene may be structurally represented as ##STR1## Isotactic polypropylene is capable of forming a highly crystalline polymer with crystalline melting points and other desirable physical properties that are considerably different from the same polymer in an amorphous, or noncrystalline, state.
A syndiotactic polymer contains principally units of alternating configurations and is represented by the structure ##STR2## A polymer chain showing no regular order of repeating unit configurations is an atactic polymer. In commercial applications, a certain percentage of atactic polymer is typically produced with the isotactic form. It is highly desirable to control the atactic form at a relatively low level.
A polymer with recurring units of opposite configuration is an isotactic stereoblock polymer and is represented by ##STR3## This latter type, the stereoblock polymer, has been successfully produced with metallocene catalysts as described in U.S. Pat. No. 4,522,982.
It may also be possible to produce true block copolymers of isotactic and atactic forms of polyolefins, especially polypropylene.
A system for the production of isotactic polypropylene using a titanium or zirconium metallocene catalyst and an alumoxane cocatalyst is described in "Mechanisms of Stereochemical Control in Propylene Polymerization with Soluble Group 4B Metallocene/Methyalumoxane Catalysts," J. Am. Chem. Soc., Vol. 106, pp. 6355-64, 1984. The article shows that chiral catalysts derived from the racemic enantiomers of ethylene-bridged indenyl derivatives form isotactic polypropylene by the conventional structure predicted by an enantiomorphic-site stereochemical control model. The meso achiral form of the ethylene-bridged titanium indenyl diastereomers and the meso achiral zirconocene derivatives, however, produce polypropylene with a purely atactic structure.
Further studies on the effects of the structure of a metallocene catalyst on the polymerization of olefins was reported in "Catalytic Polymerization of Olefins," Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, pp. 271-92, published by Kodansha Ltd., Tokyo, Japan, 1986. In this article, the effects of the chiralities, steric requirements and basicities of ligands attached to soluble titanium and zirconium metallocene catalysts on the polymerization and copolymerization of propylene and ethylene were reviewed. The studies revealed that the polymerization rates and molecular weights of the polymers obtained in the polymerization of ethylene with a zirconocene catalyst vary according to the basicity and steric requirements of the cyclopentadienyl groups. The effects of ligands also contributed to the synthesis of polypropylenes with novel microstructures and high density polyethylenes with narrow and bimodal molecular weight distributions.
The present invention relates to discoveries made as to varrying the bridge structure and substituents added to the cyclopentadienyl rings in a metallocene catalyst on the polymerization of propylene and high alpha-olefins. In particular, it was discovered that by varying these components, the physical properties of the polymer may be controlled.